1. Field of Invention
This invention relates generally to communications, and more particularly, to a method and apparatus for multi-line digital subscriber line communications.
2. Description of the Related Art
North American Integrated Service Digital Network (ISDN) Standard, defined by the American National Standard Institute (ANSI), regulates the protocol of information transmissions over telephone lines. In particular, the ISDN standard regulates the rate at which information can be transmitted and in what format. ISDN allows full duplex digital transmission of two 64 kilo bit per second data channels. These data rates may easily be achieved over the trunk lines, which connect the telephone companies' central offices. The problem lies in passing these signals across the subscriber line between the central office and the business or residential user. These lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud.
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as xDSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. An xDSL modem operates at frequencies higher than the voiceband frequencies, thus an xDSL modem may operate simultaneously with a voiceband modem or a telephone conversation. Currently there are over ten discrete xDSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there are at least two possible line codes, or modulation protocols, discrete multi-tone (DMT) and carrierless AM/PM (CAP).
XDSL modems are typically installed in pairs, with one of the modems installed in a home and the other modem, a.k.a. line card in the telephone companies central office (CO) switching office servicing that home. This provides a direct dedicated connection to the home from a line card at the central office on which the modem is implemented through the subscriber line or local loop. Each installation represents a sizeable expense in hardware and service labor to provision the central office and home. The expense may not always be amortized over a sufficient period of time due the relentless introduction of new and faster xDSL standards each of which pushes the performance boundaries of the subscriber line in the direction of increasing bandwidth and signal integrity. As each new standard involves, modems must typically be replaced to upgrade the service.
What is needed is a less rigid signal processing architecture that supports scalability of resources, and allows a more flexible hardware response to existing and evolving XDSL standards.